The present invention relates to fluid fittings and, more particularly, to a fitting adapted to be connected to a tube by swaging.
Fittings are commonly used to connect metal tubes to each other for use in a variety of applications, such as in the aerospace industry to convey fuel, hydraulic control fluids and the like in an aircraft or space vehicle. In these applications, it is critical that there be a secure connection between the fitting and the tubes in order to withstand vibration and other adverse conditions without failure. In most instances of failure, the tube cracks and ruptures either within the fitting or at the point where the tube enters the fitting. In other instances, the fitting itself will fail.
Various fittings have been developed in the past to connect tubes to each other. In one type of fitting, which is externally swaged, a curved or irregular configuration is provided on the outer surface of the fitting, with the inner surface of the fitting being relatively smooth and cylindrical. After swaging, the irregular configuration is transferred to the inner surface of the fitting which causes the tube to deflect and conform to the irregular configuration. An example of a fitting of this type is shown in U.S. Pat. No. 3,848,451.
In another type of fitting, which is swaged internally, the inner surface of the fitting has a plurality of axially spaced annular grooves into which the material of the tube is deformed by the swaging tool. The annular grooves in these fittings may have sidewalls that are either perpendicular to or inclined with respect to the longitudinal axis of the fitting. Examples of this type of fitting are disclosed in U.S. Pat. No. 3,711,132 and U.S Pat. No. 4,289,340.
Problems have existed in the past with fittings of the types described above. For example, these fittings tend to develop areas of high stress concentration at the point where the material of the tube is swaged into the grooves or irregular configuration of the fitting. These stress concentrations are major contributors to fitting or tubing fatigue failure under flexure and high operating pressures in the tubing. Another problem with these fittings is their relatively large size and weight. Typically, the tube holding power or tube retention capability of the fitting is determined by the number of annular grooves or the nature of the irregular configuration of the fitting, depending upon which one is used. In either case, the holding power is limited by the development of the undesirable stress concentrations, and to increase the holding power, considerable additional length of the fitting is required. As a result, most fittings in use today are relatively large and heavy. Large fittings pose additional problems, since they usually cannot be used in tight or difficult-to-access areas. Heavy fittings are equally troublesome because they add undesirable weight. This is especially undesirable in aircraft applications where the weight of the aircraft must be as low as possible.
Other problems encountered with known fittings is the development of fretting between the tube and the fitting when subjected to bending or rotary flexure tests. When this fretting, which is caused by microscopic axial movement of the tube with respect to the fitting, becomes too great, the fitting or the tubing, or both, will fail.
Yet another disadvantage of the known fittings is the inability to accurately position the fitting within the dies of the swaging tool in a reliable and repeatable fashion. When the fitting is not properly positioned within the dies, a tolerance stack up may occur and cause variations in the degree of swaging. When the fittings are not swaged in the same and correct position each time, the reliability of the swaged joint suffers.
Accordingly, there has existed a definite need for a swagable fitting that has a higher tube retention capability while at the same time being smaller in size and lighter in weight than the known fittings. There has further existed a need for a fitting that minimizes fretting and that can be accurately positioned within the dies of a swaging tool in a reliable and repeatable fashion. The present invention satisfies these and other needs and provides further related advantages.